The versatile experimental organism Drosophila melanogaster is presented as a model system in which to study the role of steroid hormones and gene expression in nervous system reorganization during metamorphosis. In the nervous systems of both vertebrates and invertebrates, the action of steroid hormones results in neurogenesis, cell death, and/or changes in neuronal structure. In general, steroid hormones are believed to exert their effects by altering gene expression at the transcriptional level. Thus, it is hypothesized that the steroid molting hormone ecdysterone orchestrates Drosophila CNS metamorphosis and that it does so primarily by modifying gene expression in the nervous system. The proposed experiments to test this hypothesis include classical (mutational) and molecular genetic (gene cloning) approaches, and make use of both standard histological and as well as modern immunohistochemical techniques. Initially, the maturation of the Drosophila CNS during metamorphosis will be described at the light microscopic level, yielding a staging system which will be used in subsequent analyses of mutants and in vitro CNS development. The dependence of CNS metamorphosis on ecdysterone will be determined by examining the CNS in animals which are ecdysterone-deficient an by culturing the CNS with and without the hormone. Next, the role of a known ecdysterone-inducible regulatory locus will be evaluated by examining the CNS in mutants and genetic mosaics, and by a direct search for ecdysterone-induced transcripts using cloned probes from this locus. An in vivo culture system peculiar to insects is proposed and will be tested as a novel screening method for developmental mutants likely to be required in the CNS for its normal maturation during metamorphosis. Finally, a scheme is outlined for isolating genes regulated by ecdysterone in the CNS. Genomic and cDNA libraries will be screened by differential- and subtraction-hybridization methods in order to isolate genes which are repressed or induced by the hormone. One of the hormone- inducible genes will be characterized in detail, by determining its developmental and tissue specificity and by DNA sequence analysis. A compete understanding of human neurological disorders, especially those with hereditary bases, will ultimately require the elucidation of basic cellular and molecular mechanisms of neural development. By using the sophisticated genetics of a simple insect to unravel naturally occurring, hormone-mediated nervous system reorganization, insights into human neurological disorders may emerge.